Since presenting organic light-emitting diodes and solar cells in 1989 [C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913, (1987)], components made of organic thin films have been the subject of intensive research. Such films possess advantageous properties for the mentioned applications, such as e.g. efficient electroluminescence for organic light-emitting diodes, high absorption coefficients in the visible light range for organic solar cells, inexpensive production of the materials and manufacture of the components for simplest electronic circuits, amongst others. The use of organic light-emitting diodes for display applications already has commercial significance.
The performance characteristics of (optoelectronic) electronic multilayered components were determined from the ability of the layers to transport the charge carriers, amongst others. In the case of light-emitting diodes, the ohmic losses in the charge transport layers during operation are associated with the conductivity which, on the one hand, directly influences the operating voltage required, but, on the other hand, also determines the thermal load of the component. Furthermore, depending on the charge carrier concentration in the organic layers, bending of the band in the vicinity of a metal contact results which simplifies the injection of charge carriers and can therefore reduce the contact resistance. Similar deliberations in terms of organic solar cells also lead to the conclusion that their efficiency is also determined by the transport properties for charge carriers.
By doping hole transport layers with a suitable acceptor material (p-doping) or electron transport layers with a donor material (n-doping), respectively, the density of charge carriers in organic solids (and therefore the conductivity) can be increased substantially. Additionally, analogous to the experience with inorganic semiconductors, applications can be anticipated which are precisely based on the use of p- and n-doped layers in a component and otherwise would be not conceivable. The use of doped charge-carrier transport layers (p-doping of the hole transport layer by admixture of acceptor-like molecules, n-doping of the electron transport layer by admixture of donor-like molecules) in organic light-emitting diodes is described in U.S. Pat. No. 5,093,698.
Hitherto, the following approaches are known for improving the conductivity of organic vapour-deposited layers:
Increasing the charge carrier mobility by
using electron transport layers consisting of organic radicals (U.S. Pat. No. 5,811,833),
generating highly ordered layers which allow for an optimal overlap of the π orbital of the molecules,
increasing the density of the mobile charge carriers by
cleaning and careful treatment of the materials to avoid formation of charge-carrier adherence sites,
doping organic layers with
inorganic materials (alkaline metals: J. Kido et al., U.S. Pat. No. 6,013,384; J. Kido et al., Appl. Phys. Lett. 73, 2866 (1998), oxidants such as iodine, SbCl5 etc.)
organic materials (TNCQ: M. Maitrot et al., J. Appl. Phys., 60 (7), 2396-2400 (1986), F4TCNQ: M. Pfeiffer et al., Appl. Phys. Lett., 73 (22) 3202 (1998), BEDT-TTF: A. Nollau et al., J. Appl. Phys., 87 (9), 4340 (2000), naphthalenedicarboxylic amides: M. Thomson et al., WO03088271, cationic dyes: A. G. Werner, Appl. Phys. Lett. 82, 4495 (2003))
organometallic compounds (metallocenes: M. Thomson et al., WO03088271)
metal complexes (Ru0 (terpy)3: K. Harada et al., Phys. Rev. Lett. 94, 036601 (2005).
While sufficiently strong, organic dopants already exist for p-doping (F4TCNQ), only inorganic materials, e.g. cesium, are available for n-doping. Through usage thereof, it has already been possible to achieve the improvement of the performance parameters of OLEDs. Thus, a dramatic reduction of the operating voltage of the light-emitting diode is reached by doping the hole transport layer with the acceptor material F4TCNQ (X. Zhou et al., Appl. Phys. Lett., 78 (4), 410 (2001)). It is possible to achieve a similar success by doping the electron-transporting layer with Cs or Li (J. Kido et al., Appl. Phys. Lett., 73 (20), 2866 (1998); J.-S. Huang et al., Appl. Phys. Lett., 80, 139 (2002)).
For a long time, a major problem in n-doping was that only inorganic materials were available for this process. However, using inorganic materials has the drawback that the atoms or molecules used can easily diffuse in the component due to their small size and thus can impede a defined production of sharp transitions from p-doped to n-doped areas, for example.
In contrast, the diffusion should play an inferior role when using large, space-filling, organic molecules as dopants as circuit crossing processes are only possible when higher energy barriers are overcome.
From WO 2005/086251 A2, the use of a metal complex as n-dopant for doping an organic semiconductor matrix material to change the electrical properties thereof is known in which the connection relative to the matrix material represents an n-dopant. In this, it is proposed to employ, as the dopant compound, a neutral metal complex rich in electrons and having a central atom as preferably a neutral or charged transition metal atom with a number of valence electrons of at least 16.
It has been known for many years, in particular in the case of organic polymeric semiconductor materials, that an effective electron transfer from a dopant (for example sodium) to the organic matrix (for example polyacetylene) is only possible if the difference between HOMO energy level (=ionisation potential) of the dopant and the LUMO energy level (=electron affinity) of the matrix is as small as possible.
Ultraviolet photoelectron spectroscopy (UPS) is the preferred method to determine the ionisation potential (e.g. R. Schlaf et al., J. Phys. Chem. B 103, 2984 (1999)). A related method, inverse photoelectron spectroscopy (IPES), is used to determine electron affinities (e.g. W. Gao et al., Appl. Phys. Lett. 82, 4815 (2003)), however, this is not as well-established. Alternatively, the solid state potentials can be estimated via electrochemical measurements of oxidation potentials Eox or reduction potentials Ered, respectively, in the solution, e.g. by cyclic voltammetry (CV) (e.g. J. D. Anderson, J. Amer. Chem. Soc. 120, 9646 (1998)). Several papers state empirical formulae for converting the electrochemical voltage scale (oxidation potentials) to the physical (absolute) energy scale (ionisation potentials), e.g. B. W. Andrade et al., Org. Electron. 6, 11 (2005); T. B. Tang, J. Appl. Phys. 59, 5 (1986); V. D. Parker, J. Amer. Chem. Soc. 96, 5656 (1974); L. L. Miller, J. Org. Chem. 37, 916 (1972), Y. Fu et al., J. Amer. Chem. Soc. 127, 7227 (2005). No correlation of reduction potential and electron affinity is known as electron affinities can only be measured with difficulty. Hence, the electrochemical and physical energy scales are converted into each other in a simplified way by means of IP=4.8 eV+e*Eox (vs. ferrocene/ferrocenium) and EA-4.8 eV+e*Ered (vs. ferrocene/ferrocenium), respectively, as described in B. W. Andrade, Org. Electron. 6, 11 (2005) (also see ref. 25-28 therein). The conversion of different standard potentials and redox pairs, respectively, is described in A. J. Bard, L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2nd Edition 2000, for example. Thus, it results from the depiction above that the exact determination of all energy values currently is not possible and that the presented values can only be construed as benchmarks.
The dopant functions in n-doping as an electron donor and transmits electrons to a matrix which is characterised by a sufficiently high electron affinity. That means that the matrix is reduced. By means of the transfer of electrons from the n-dopant to the matrix, the charge carrier density of the layer is increased. To which extent an n-dopant is able to release electrons towards a suitable matrix with electron affinity and thereby increase the charge carrier density and, as a consequence thereof, the electroconductivity depends in turn on the relative position of the HOMO of the n-dopant and the LUMO of the matrix in relation to one another. If the HOMO of the n-dopant is positioned above the LUMO of the matrix with electron affinity, an electron transfer can take place. If the HOMO of the n-dopant is arranged beneath the LUMO of the matrix with electron affinity, an electron transfer can likewise take place, provided that the energy difference between the two orbitals is sufficiently small to allow for a certain thermal population of the higher energy orbital. The smaller this energy difference, the higher the conductivity of the resulting layer should be. However, the highest conductivity can be anticipated in the case that the HOMO level of the n-dopant is higher than the LUMO level of the matrix with electron affinity. The conductivity can be measured conveniently and is a measure of how well the electron transmission from donor to acceptor functions, provided that the charge carrier mobilities of different matrices can be compared.
The conductivity of a thin-film sample is measured by the 2 point method. In this, contacts made from a conductive material, e.g. gold or indium tin oxide, are applied to a substrate. Thereafter, the thin film to be examined is applied to a large surface area of the substrate such that the contacts are covered by the thin film. After applying a voltage to the contacts, the current subsequently flowing is measured. Starting from the geometry of the contacts and the layer thickness of the sample, the conductivity of the thin-film material results from the resistance thus determined. The 2 point method is admissible when the resistance of the thin film is substantially higher than the resistance of the leads or the contact resistance. This is in the experiment ensured by a sufficiently large distance of the contacts and thus, the linearity of the voltage-current characteristic can be checked.